Oxidative Dehydration of Glycerol to Acrylic Acid over Vanadium

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Oxidative Dehydration of Glycerol to Acrylic Acid over Vanadium Substituted Cesium Salts of Keggin-type Heteropolyacids Xiukai Li, and Yugen Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00213 • Publication Date (Web): 20 Mar 2016 Downloaded from http://pubs.acs.org on March 21, 2016

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ACS Catalysis

Oxidative Dehydration of Glycerol to Acrylic Acid over Vanadium Substituted Cesium Salts of Keggin-type Heteropolyacids Xiukai Li, Yugen Zhang*

Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669, Singapore.

ABSTRACT Acrylic acid is one of the most desired products from bio-glycerol. In this work, we demonstrated that Keggin-type heteropoly compounds (HPCs) act as efficient bifunctional catalysts for the one-step conversion of glycerol to acrylic acid in gas phase. This is the first example of HPCs as catalysts for selectively conversion of glycerol to acrylic acid. It was found that introducing of vanadium species into the secondary structure of the cesium salts of H3PMo12O40 and H3PW12O40 heteropolyacids is beneficial for acrylic acid formation. The combination of H0.1Cs2.5(VO)0.2PMo12O40 and H0.1Cs2.5(VO)0.2PW12O40 as solid solutions resulted in variations in the oxidization ability and surface acidity of the catalysts and, consequently, notably improved selectivity for acrylic acid. Up to 60% yield to acrylic acid was

achieved

over

the

catalyst

with

the

formula

of

H0.1Cs2.5(VO)0.2(PMo12O40)0.25(PW12O40)0.75. The catalyst also showed good resistance to coke deposition and long term stability due to the salt feature. It is deduced that the synergism between the Keggin anions and the cesium/vanadium entities at the secondary structure determined the overall catalyst performance.

Keywords: glycerol, acrylic acid, heteropolyacids, acrolein, gas phase reaction

INTRODUCTION With the increasing demands for green and renewable energy, biodiesel economy has boomed from the last decade and the production of biodiesel is forecasted to reach 37 million metric tons in 2020.1 Glycerol is coproduced in 10 wt% with biodiesel in the transesterification of plant oil or animal fat with methanol.2,3 Thus, the growth of biodiesel has led to the surplus production and the decline price of glycerol. The better utilization of 1

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glycerol will be of great importance for the biodiesel economy. The abundant bio-generated glycerol provides additional opportunity for the usage of renewable resource in synthetic chemistry.4-6 Considerable efforts have been devoted to the transformation of glycerol to value added chemicals, and various methods including oxidation,7,8 hydrogenolysis,9,10 reforming,11-13 and dehydration 1,14,15 have been developed for this purpose. Acrolein and acrylic acid (AA) are important bulk chemicals and they are two direct derivatives from bio-generated glycerol.16,17 Acrolein can be derived from glycerol dehydration at > 90% yield over solid acid catalysts such as metal oxides,18,19 zeolites,20,21 heteropolyacids,22-25 and phosphates.16,26 The synthesis of acrylic acid from glycerol with acrolein as the intermediate can be achieved by the two-step tandem reaction (Scheme 1) with an acid catalyst and an oxidation catalyst loaded separately in two in-series beds.16,27,28 Moand V-based catalysts are efficient for the oxidation of acrolein to acrylic acid in the second step and up to 70% overall yield of acrylic acid could be achieved.28 More recently, a new two-step tandem reaction system for glycerol conversion to acrylic acid via allyl alcohol intermediate has also been reported.29 Although the two-step tandem reaction systems could give high yield of acrylic acid, the oxidative dehydration of glycerol to acrylic acid in a single bed over a multifunctional catalyst is highly desired for the advantages of simple reactor design, less investment input, and lower operation costs. Several catalysts with both acidic and redox characteristics have so far been demonstrated for the one-step oxidative dehydration of glycerol to acrylic acid; however, in most cases the yield to acrylic acid was less than 30%.30-35 Recently, it has been reported that around 50% yields of acrylic acid could be achieved from glycerol in one step over the WVMo36 and WVNb37,38 catalysts. The WVMo catalyst partially decomposed after 28 h reaction and the acrylic acid yield decreased to 42%,36 while the WVNb catalysts can be stable up to 35 h.37 Phosphoric acid modified WVNb catalyst promoted the yield of acrylic acid to 60%, while the long term stability of this catalyst is not clear.38 Catalyst deactivation due to coke formation is the major challenge in both glycerol dehydration to acrolein and glycerol oxidative dehydration to acrylic acid. Approaches have been tested to solve this issue by co-feeding with oxygen,26,39 metal doping,23 and manipulating the catalyst texture,40,41 however, only limited success was achieved.

2


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ACS Catalysis

d hy de

n tio ra

lys ta ca

t

ox id at io n

ca ta ly s

n tio da i ox

t

st ly ta a c

Scheme 1. The two-step and the one-step conversion of glycerol to acrylic acid. Keggin-type heteropoly compounds (HPCs) are a unique type of materials which have long been used as acidic and/or redox catalysts.42-45 However, up to date HPCs were used mainly as acidic catalysts for glycerol dehydration to acrolein1,17,23,24,28 and their redox properties for glycerol oxidative conversion have been less explored. In this work, we proposed to use transition metal substituted cesium salts of Keggin-type heteropolyacids (HPAs) as bifunctional catalysts for glycerol conversion to acrylic acid in one step. It turned out that up to 60% yield of acrylic acid could be achieved by tuning the elemental composition of the catalysts. To our knowledge, this is the first example of HPCs catalysts for glycerol conversion with acrylic acid as the major product. This finding could be significant, as it may disclose a new type of catalysts for the one-step conversion of glycerol to acrylic acid at high yield, and will give deeper insight into the catalytic properties of HPCtype compounds.

EXPERIMENTAL SECTION All starting materials are commercially available and were used as received, unless otherwise indicated. Glycerol (> 99%) and Cs2CO3 (99.9%) were purchased from Sigma. H3PW12O40 and H3PMo12O40 were purchased from Merck. Other regents not stated were purchased from Sigma or Merck. Catalyst Preparation. The vanadium substituted cesium salts of HPAs were prepared by a precipitation method. In a typical procedure, certain amount of H3PMo12O40 and/or H3PW12O40 was dissolved in 30.0 ml of distilled water under stirring. Then stoichiometric amounts of aqueous solutions of vanadyl chloride (0.1 mol L-1) and cesium carbonate (0.15 3


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mol L-1) were added dropwise. Upon the addition of cesium carbonate, a yellow precipitate was observed immediately. The suspension was evaporated to dryness, and then the powder sample was heated at 100 °C in air for 12 h. The catalysts were pressed, crushed, and sieved to 35 – 60 mesh for use. The H0.5Cs2.5PMo12O40 and H0.5Cs2.5PW12O40 samples are denoted as CsPMo and CsPW, respectively. The H0.1Cs2.5(VO)0.2PMo12O40 and H0.1Cs2.5(VO)0.2PW12O40 samples are denoted as Cs(VO)0.2PMo and Cs(VO)0.2PW, respectively. The solid solutions of H0.1Cs2.5(VO)0.2PMo12O40

and

H0.1Cs2.5(VO)0.2PW12O40

are

denoted

as

Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1). Catalyst Characterization. Powder X-ray Diffraction (XRD) was conducted using a Bruker automatic diffractometer (Bruker D8 discover GADDS) with monochromatized CuKα radiation (λ = 0.15406 nm) at a setting of 30 kV. The BET surface areas and pore structures were measured by Micromeritics ASAP 2020. The samples were degassed at 200 °C for 4 h before N2 adsorption. The SEM images were obtained with a JEOL JSM 7500 scanning electron microscope. FTIR spectra were collected on the PerKinElmer Spectrum 100 FT-IR Spectrometer. Temperature-programmed reduction (TPR) was carried out in the temperature range of 100 – 800 ºC. The sample (50 mg) was reduced in 5% H2/Ar (30 mL min-1) at a heating rate of 5 ºC min-1. Gas Phase Conversion of Glycerol to Acrylic Acid.

Catalytic performance for

glycerol conversion to acrylic acid was evaluated in a quartz fixed-bed microreactor (Φ = 6 mm) with a continuous reactant down flow. Typically, 0.2 g of catalyst (35 – 60 mesh) was charged into the reactor. The reactant (20 wt% glycerol in water) was fed by a high accuracy syringe pump at a rate of 0.5 mL h-1 (WHSV: 0.5 h-1). The carrier gas was 5% O2/He (flow rate 20 mL min-1). On-line gas chromatography (Shimadzu GC2014) equipped with FID and TCD was used to analyze the outlet effluent after the system was stabilized for 1 h at each temperature point. A capillary column (HP FFAP, 30 m × 0.32 mm, 0.25 µm) was used to separate the organic components, and a packed carbon column (3 mm × 300 mm) was used to separate CO and CO2.

RESULTS AND DISCUSSION H3PW12O40 and H3PMo12O40 are the most extensively studied HPA-type catalysts as they are cheap and commercially available in large amount. Comparing to H3PW12O40, H3PMo12O40 has stronger oxidation ability and weaker acidity due to the weaker Mo-O bonding (vis W-O bonding).46 Molybdenum HPAs and their cesium salts were used as acid or 4


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ACS Catalysis

oxidation catalysts,42,45,47,48 while tungsten HPAs and their cesium salts were used mainly as acid catalysts.22,23,28,49-51 In our study, the H0.5Cs2.5PMo12O40 (CsPMo) sample was firstly applied for glycerol oxidative dehydration in gas phase (Figure 1). Full glycerol conversion was achieved at 340 °C and the major products were acrolein (53.1% yield), COx (30.5% yield), acetic acid (7.0% yield), and acrylic acid (4.9% yield). Acetaldehyde was observed as the minor product and it was included in others. Hydroxyl acetone is a possible product in glycerol dehydration however it was not detected in our system. It seems that CsPMo is active but less selective for acrylic acid. In order to further improve the selectivity for acrylic acid, various transition metals were introduced into the secondary structure of CsPMo. The substitution of transition metals generally enhanced the oxidation ability of CsPMo as reflected by the decreased selectivity to acrolein while increased amount of the total oxidized products (COx). The selectivity for acrylic acid decreased with the substitution of Mn2+ and Fe3+, and increased slightly to 6.0 – 10.7% with the substitution of Cr3+, Co2+, Ni2+, and Cu2+. With the substitution of VO2+, the selectivity for acrylic acid increased notably from 4.9 to 23.2%. These results demonstrated that introducing VO2+ to the secondary structure of HPCs is beneficial for the AA formation. It was reported that vanadium species in HPC-type catalysts are also essential for the synthesis of methacrylic acid from methacrolein oxidation,45 acrylic acid from propane oxidation,42 and maleic anhydride from n-butane oxidation.51 In addition, vanadium can also be introduced into the PMo12O403- anion by replacing one or more of the molybdenum atoms. In order to gain better understanding on the the role of vanadium substitution on the catalyst performance in glycerol conversion, a H1.5Cs2.5PMo11VO40 (CsPMoV) sample with vanadium substituted at the primary structure of the Keggin anion was also tested (Figure 1). However, the introduction of one vanadium atom to the PMo12O403- anion of CsPMo resulted in enhanced formation of COx. Vanadium substituted at the primary structure of the Keggin anion would result in the generation of more reactive lattice oxygen and consequently produce more over-oxidized products. Moreover, the vanadium atoms at the primary structure are not stable and could be segregated as vanadium oxide at high reaction temperature.42,52 It is therefore deduced that a small amount of vanadium components existed at the secondary structure of the Keggin-type HPCs is more favorable for AA formation than vanadium substituted at the primary structure. Introducing of the reactive vanadium species at the secondary structure may change the redox property of the catalyst and generate new active sites on sample surface.42 Moreover,

5


ACS Catalysis

vanadium substitution at the secondary structure of HPAs could enhance the salt feature of the HPCs and consequently increase the thermal stability of the catalysts. 100

Conv., sel. / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 Conv. Sel. AA Sel. Acr Sel. AcOH Sel. COx Sel. Others

60

40

20

0 M = None VO2+ Cr3+ Mn2+ Fe3+ Co2+ Ni2+ Cu2+

CsM0.2PMo

CsPMoV

Figure 1. Glycerol conversion over CsM0.2PMo (M = VO2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, and Cu2+) and CsPMoV catalysts. Reaction conditions: catalyst, 0.2 g, 35 – 60 mesh; temperature, 340 °C; carrier gas, 5% O2/He, 20 mL min-1; feed, 20 wt% glycerol in H2O, 0.5 mL h-1; WHSV: 0.5 h-1. Acr: acrolein; AA: acrylic acid; AcOH: acetic acid; COx: CO and CO2. Data collected at the 1 h time point. As promotion of acrylic acid formation by introducing VO2+ to the secondary structure of CsPMo is feasible, we further varied the amount of VO2+ to investigate its influences on glycerol conversion to AA (Figure 2). The promotion effect for AA formation would be significant when the amount of VO2+ exceeds 0.2. The best AA selectivity (23.2%) was achieved over Cs(VO)0.2PMo. However, the over-oxidized products (COx) were also increased (26% – 40%) due to the strong oxidation ability of the Cs(VO)xPMo catalysts. The effect of VO2+ substitution on acrylic acid formation was also investigated for the cesium salt of H3PW12O40. The CsPW sample without VO2+ substitution afforded 88.5% selectivity for acrolein however only 3.0% selectivity for acrylic acid. This is in good agreement with the fact that the sample has relatively stronger acidity while weaker oxidation ability as aforementioned. Similarly, in an earlier report, exceeds 70% selectivity for acrolein was achieved over a supported CsPW catalyst.28 When VO2+ was introduced into the secondary structure of CsPW, the selectivity for the total oxidized products (COx) also increased remarkably at the VO2+ amount of 0.1 and 0.2, signifying enhanced oxidation ability of the catalyst. The selectivity for acrylic acid improved from 3.0% of non-substituted CsPW to 6


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11.4% of Cs(VO)0.2PW, meanwhile the selectivity for acrolein decreased from 88.5% to 46.3%. Thus, Cs(VO)0.2PMo has stronger oxidation power and gave higher AA yield and higher total oxidized products, while Cs(VO)0.2PW has stronger acidity and weaker oxidation ability and gave higher acrolein yield. The elemental compositions for both the secondary structure and the primary structure of the HPCs determine their catalytic performance. 100

Conv., sel. / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

80

Conv. Sel. AA Sel. Acr Sel. AcOH Sel. COx Sel. Others

60

40

20

0

x=0

0.1

0.2 0.25 0.5

x=0

Cs(VO)xPMo

0.1

0.2

0.5

Cs(VO)xPW

Figure 2. Glycerol conversion over serial Cs(VO)xPMo (x = 0, 0.1, 0.2, 0.25, and 0.5) and Cs(VO)xPW (x = 0, 0.1, 0.2, and 0.5) catalysts. Reaction conditions: catalyst, 0.2 g, 35 – 60 mesh; temperature, 340 °C; carrier gas, 5% O2/He, 20 mL min-1; feed, 20 wt% glycerol in H2O, 0.5 mL h-1; WHSV: 0.5 h-1. Acr: acrolein; AA: acrylic acid; AcOH: acetic acid; COx: CO and CO2. Data collected at the 1 h time point. It is demonstrated that vanadium substitution at the secondary structure of Keggin-type HPCs like CsPMo and CsPW will increase the oxidation power of the catalysts and promote acrylic acid formation in the glycerol oxidative dehydration process. However, for the serial Cs(VO)xPMo catalysts, the total oxidized COx products is significant due to the strong oxidation ability of the Keggin oxoanion. For the serial Cs(VO)xPW catalysts, acrolein is still dominant in products due to the strong acidity and weak oxidation ability of the Keggin anion. Thus, we envisaged that it might be possible to further improve the selectivity for acrylic acid by combining Cs(VO)xPMo and Cs(VO)xPW into one sample to balance the oxidation ability and the acidity of the catalysts. Therefore, serial solid solution samples with the formula of Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) were developed for the conversion of glycerol to AA. Figure 3 depicts the XRD patterns of the solid solution catalysts. All samples show very similar diffraction pattern and the peaks at 2θ = 10.5º, 18.3º, 23.7º, 26.1º, 30.1º, 35.6º, and 38.7º are assignable to the cubic alkaline salts of HPA.42 The FT-IR spectra of samples are 7


ACS Catalysis

shown in Figure 4. All the HPC samples exhibit four characteristic IR bands at 1060 – 1080, 960 – 983, 860 – 886, and 766 – 781 cm−1, attributable to νas 42,53

νs M-Oc-M, respectively.

P-Oi,

νs M(Mo/W)=Ot, νs M-Ob-M, and

With increasing content of the PMo12O403- oxoanion, all of the four

bands shift gradually toward lower wave numbers. It is noteworthy that there is only one set of the characteristic IR bands observed for the serial Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) samples. In contrast, a mechanical mixture of Cs(VO)0.2PMo and Cs(VO)0.2PW (1: 1, mass ratio) exhibits the separated νas

P-Oi

and νs

M=Ot

bands from the two components (Figure S1).

Thus, it is suggested that the serial Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) samples are single phase solid solutions instead of the mechanical mixtures of Cs(VO)0.2PMo and Cs(VO)0.2PW. We envision that the co-precipitation method adopted for sample preparation enabled the well mixing of the two HPAs at molecular level to form the solid solutions. All samples exhibit specific surface area in the range of 55 – 107 m2 g-1 and the values decrease with increasing of PMo12O403- oxoanion content in the samples (Table S1). The SEM images (Figure S2) of representative samples depict that the catalyst contains particles of about 50 – 100 nm.

222 211 310 400 322

x=1 0.9 0.75 0.5 0.25

Intensity / a.u.

110

0.1 0.05 0 10

20

30

40

50

60

2θ / °

Figure 3. The XRD patterns of serial Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) catalysts.

x=1 Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.9 0.75 0.5 0.2 0.1 0.05 0 M-Ob-M

P-Oi

M-Oc-M

M=Ot

1200

1100

1000

900

800

700

600

-1

Wave number / cm

Figure 4. The FT-IR spectra of serial Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) catalysts. 8


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Depicted in Figure 5 are the H2-TPR profiles of the serial Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) samples. One main reduction peak is observed for all of the samples within the temperature range of 590 – 715 °C. In terms of the peak temperature, the Cs(VO)0.2PMo sample is more reducible than Cs(VO)0.2PW. With increasing content of PW12O403- oxoanion in the samples, the peak maximum gradually shifts to the higher temperature end. In terms of the onset temperature, the lattice oxygen of the Cs(VO)0.2PMo sample is more reactive than that of Cs(VO)0.2PW. As all of the samples have the same cations at the secondary structure, the elemental compositions at the primary structure (i.e., the Keggin anion) should account for the observed redox property of the samples. Due to the weaker bonding with Mo, the bridge oxygen and terminal oxygen in the PMo12O403- oxoanion are more reactive than those oxygen atoms in the PW12O403- oxoanion and this is also in agreement with the observed TPR behaviours. As the partial oxidation reaction over metal oxide catalysts usually follow the proposed Mars-van Krevelen mechanism, the higher reducibility of the lattice oxygen atoms indicates a stronger oxidation ability of the catalyst.

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

x=1 0.9 0.75 0.5 0.25 0.1 0 350

400

450

500

550

600

650

700

750

T / ºC

Figure 5. TPR profiles of Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) catalysts. Reaction conditions: catalyst, 50 mg, 35 – 60 mesh; temperature, 100 – 800 °C at a rate of 5 °C min-1; carrier gas, 5% H2/Ar, 30 mL min-1. The surface acidities of the serial Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) samples were measured by in situ FT-IR using pyridine as a probe (Figure 6). Pyridine adsorption was performed at room temperature (30 °C) and then the system was evacuated at 30 °C and 150 °C respectively. In the case when the system was evacuated at 30 °C (Figure 6A), IR bands related to chemisorbed pyridine were observed at 1439, 1537, and 1486 cm-1, attributable to the pyridine cations on the Lewis acid (L acid) sites, the Brønsted acid (B acid) sites, and both L and B acid sites, respectively. The relative acidities of samples can be 9


ACS Catalysis

roughly compared on the basis of the band areas. It is clear that the acidity (the amount of pyridine uptake) decreases with the increasing of PMo12O403- content in the samples. It is also noted that the 1439 cm-1 band (L acid sites) diminished more rapidly than the 1537 cm-1 band (B acid sites), owing to that the Cs(VO)0.2PMo sample possesses less amount of Lewis acid sites than Cs(VO)0.2PW. When the system was evacuated at 150 °C (Figure 6B), the band areas also decrease with the increasing of PMo12O403- content. The 1439 cm-1 band completely disappeared for all of the samples whereas the 1486 cm-1 band remained, suggesting that the strength of Lewis acid is weaker than that of Brønsted acid on these sample surfaces. A

-1

1537 cm

-1

1486 cm

B

-1

1439 cm

-1

1537 cm

-1

1486 cm

0.25 0.5 0.75

-1

1439 cm

X=0

Absorbance

X=0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.25 0.5 0.75 1

1

1550

1500

1450

1550

1400 -1

1500

1450

Wave number / cm

Wave number / cm

1400 -1

Figure 6. In situ FT-IR spectra of pyridine adsorption on the surface of serial Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) catalysts. (A) Pyridine adsorption at 30 °C (5 min) followed by evacuation at 30 °C (10 min), and (B) heating to 150 °C and evacuation at 150 °C for 10 min.

The serial Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) samples were then evaluated for glycerol oxidative dehydration to AA with the results shown in Figure 7. Full glycerol conversion was achieved over all samples at reaction temperature of 340 °C. Over the Cs(VO)0.2PW sample, the selectivity for acrolein and acrylic acid were 45.5% and 11.2%, respectively. With increasing content of the PMo12O403- oxoanion in the solid solution samples, the selectivity for acrolein decreased remarkably meanwhile the selectivity for AA increased accordingly. Acetic acid and COx were observed as major by-products. Acetaldehyde was still observed as the minor product and hydroxyl acetone was not detected. The best selectivity for AA (ca. 60%) was maximized at the PMo12O403- content of 0.25. Herein, the yield of AA (60%) is greater than the sum of yields of both AA and acrolein obtained over the two individual catalysts (x = 0 and 1). Further increase the content of the PMo12O403- oxoanion led to the lower yield of AA but notably increased amount of COx. Thus, under the current reaction 10


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conditions, up to 60% AA yield could be obtained from glycerol oxidative dehydration over the Cs(VO)0.2(PMo)0.25(PW)0.75 catalyst, and this is one of the best results as compared with those in literature reports.36-38 It follows that the synergism between the Cs+ and VO2+ cations and the PMo12O403- and PW12O403- oxoanions determines the redox and acidic properties of the catalyst and therefore the selectivity for AA. In order to gain more understanding on this complex catalytic system, a H0.1Cs2.5(VO)0.2PMo6W6O40 (Cs(VO)0.2PMoW) sample with both Mo and W atoms substituted at the Keggin oxoanion was prepared and evaluated (Table 1). However, the sample is less selective for AA but more selective for COx as compared to the Cs(VO)0.2(PMo)0.5(PW)0.5 solid solution sample. It is likely that Cs(VO)0.2PMo and Cs(VO)0.2PW provide different active sites in the process of AA formation and their synergism determines the overall product distribution. 100

Conv., sel. / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Sel. AA Sel. Acr Sel. COx Sel. AcOH Conv.

80 60 40 20 0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

X

Figure 7. Glycerol conversion over serial Cs(VO)0.2(PMo)x(PW)1-x (x = 0 – 1) catalysts. Reaction conditions: catalyst, 0.2 g, 35 – 60 mesh; temperature, 340 °C; carrier gas, 5% O2/He, 20 mL min-1; feed, 20 wt% glycerol in H2O, 0.5 mL h-1; WHSV, 0.5 h-1. Acr: acrolein; AA: acrylic acid; AcOH: acetic acid; COx: CO and CO2. Data collected at the 1 h time point. Table 1. Glycerol conversion over the vanadium substituted cesium salts of heteropolyacids. Conv. (%)

Sample

Sel. (%) Acr AcOH AA Others COx

Cs(VO)0.2(PMo)0.5(PW)0.5

100

6.9 16.5

56.6 6.8

13.2

Cs(VO)0.2PMoW

100

9.3 15.5

40.9 9.0

25.4

Reaction conditions: catalyst, 0.2 g, 35 – 60 mesh; temperature, 340 °C; carrier gas, 5% O2/He, 20 mL min-1; feed, 20 wt% glycerol in H2O, 0.5 mL h-1; WHSV: 0.5 h-1. Acr: acrolein; AA: acrylic acid; AcOH: acetic acid; COx: CO and CO2. Data collected at the 1 h time point. 11


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The operation variables such as the reaction temperature, the oxygen content and the flow rate for the carrier gas, and glycerol concentration were also studied with the best performed Cs(VO)0.2(PMo)0.25(PW)0.75 catalyst. From Figure 8, one can see that full glycerol conversion was achieved at 260 °C and the selectivity for acrolein and acrylic acid were 67.1% and 9.3%, respectively. The selectivity for acrolein decreased rapidly with the temperature rise in the range of 260 – 380 °C. In contrast the selectivity for AA increased rapidly with the increase in temperature, reaching a maximum of approximately 60% at 340 °C. A further increase in temperature led to the decreased selectivity for AA due to the over-oxidation to acetic acid, COx, and other side products as depicted in Figure 8. This result indicates that acrolein is the key intermediate and it is quickly oxidized to AA at higher temperature. When 5% O2/He was used as carrier gas at the flow rate of 20 mL min-1, approximately 60% yield of AA was achieved (Table S2). Oxygen content less than 5% or gas flow rate lower than 20 mL min-1 led to fewer amounts of total oxidized products (COx) and also lower selectivity for acrylic acid. Higher oxygen content or higher gas flow rate led to more total oxidized products and also lower selectivity for acrylic acid. The reaction was also tested at different weight hourly space velocity (WHSV) values by varying the concentration of the glycerol solution (Table S3). At WHSV value of 0.25 h-1, above 50% selectivity for acrylic acid was still achieved but the selectivity for AcOH (34%) was also significant. When the WHSV value increased to 1.0 h-1, full glycerol conversion was still achieved at 340 °C. However, the selectivity for acrylic acid decreased to 44.1% and acrolein was observed at a selectivity of 22.6%, indicating the oxygen/reactant ratio is too low to fully oxidize acrolein to AA. Thus, a WHSV value of 0.5 h-1 and using the 5% O2/He carrier gas at the flow rate of 20 mL min-1 are the preferred reaction conditions for the high yield of acrylic acid.

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100

Conv., sel. / %

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ACS Catalysis

Conv. Sel. AA Sel. Acr Sel. AcOH Sel. COx Others

80

60

40

20

0

260

280

300

320

340

360

380

400

Temperature / °C

Figure

8.

The

temperature

dependence

of

glycerol

conversion

over

the

Cs(VO)0.2(PMo)0.25(PW)0.75 catalyst. Reaction conditions: catalyst, 0.2 g, 35 – 60 mesh; carrier gas, 5% O2/He, 20 mL min-1; feed, 20 wt% glycerol in H2O, 0.5 mL h-1; WHSV: 0.5 h1

. Acr: acrolein; AA: acrylic acid; AcOH: acetic acid; COx: CO and CO2. Data collected at the

1 h time point.

The

stability

is

crucial

for

a

practical

heterogeneous

catalyst.

The

Cs(VO)0.2(PMo)0.25(PW)0.75 catalyst was therefore tested as a function of time on stream at WHSV of 0.5 h-1. As shown in Figure 9, the decrease in the yield of acrylic acid was observed after 10 hours and the value dropped from 60% to 44% in 20 hours. After 20 hours time on stream, the yield to acrylic acid decreased slowly to 38% at 56 h. The XRD analysis for the used catalyst indicates that the structure of the HPC catalyst was well retained after 60 hours reaction (Figure S3). It was noted that the colour of the catalyst changed from yellow to molybdenum blue after the reaction, indicating the catalyst especially the PMo12O403oxoanion was partially reduced. This is in agreement with the fact that the terminal and bridge oxygen atoms in the PMo12O403- oxoanion are very reactive and reducible. Most catalysts used for glycerol dehydration or oxidative dehydration suffer from the problem of deactivation

due

to

coke

deposition

on

catalyst

surface.

Thus,

the

used

Cs(VO)0.2(PMo)0.25(PW)0.75 catalyst was subjected to thermogravimetric analysis (TGA) with the result shown in Figure S4. The weight loss in the temperature range of 300 – 600 °C was less than 3%, indicating the coke deposition on sample surface is insignificant. It has been reported that coke deposition could be 10 – 20 wt% on the HPA catalysts used in glycerol dehydration under inert atmosphere.54 The co-feeding with oxygen could reduce the amount 13


ACS Catalysis

of coke deposition to certain extent.28,39 Besides the oxygen atmosphere used in the oxidative dehydration reaction, the better resistance to coke deposition of the current catalyst might also be attributed to the salt feature and the less acidity of the partially neutralized Cs(VO)0.2(PMo)0.25(PW)0.75 catalyst as compared with the totally proton-free HPAs. The salt feature of the HPC catalysts also makes it possible for catalyst regeneration at higher temperature.28 Thus, we have further demonstrated the regeneration of the catalyst with the results shown in Figure S5. The catalyst was regenerated by simple calcination in air for 4 hours after stability testing for 30 hours. After calcination, the colour of the reduced catalyst changed from blue to yellow, signifying the re-oxidation of the catalyst. The yield for acrylic acid can be basically recovered after catalyst regeneration. 100

80

AA yield / %

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60

40

20

0

0

10

20

30

40

50

60

Time / h

Figure 9. Time on stream test for glycerol oxidative dehydration to acrylic acid over the Cs(VO)0.2(PMo)0.25(PW)0.75 catalyst. Reaction conditions: catalyst, 0.2 g, 35 – 60 mesh; carrier gas, 5% O2/He, 20 mL min-1; feed, 20 wt% glycerol in H2O, 0.5 mL h-1; WHSV: 0.5 h1

.

CONCLUSION We have demonstrated that Keggin-type heteropoly compounds (HPCs) with suitable elemental compositions can be used as efficient bifunctional catalysts for the one-step conversion of glycerol to acrylic acid in gas phase. The introducing of vanadium species into the secondary structure of the cesium salts of HPAs is beneficial for acrylic acid formation. The combination of Cs(VO)0.2PMo and Cs(VO)0.2PW as solid solution catalysts resulted in systematic changes in the oxidization ability, surface acidity, and improved selectivity for acrylic acid. Up to 60% yield to acrylic acid was achieved over the catalyst Cs(VO)0.2(PMo)0.25(PW)0.75.

Because

of

the

salt

feature

14


of

the

sample,

the

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ACS Catalysis

Cs(VO)0.2(PMo)0.25(PW)0.75 catalyst showed good resistance to coke deposition and long term stability as compared with the proton-free HPAs. ACKNOWLEDGEMENTS This work was supported by the Institute of Bioengineering and Nanotechnology (Biomedical Research Council, Agency for Science, Technology and Research (A*STAR), Singapore), Biomass-to-Chemicals Program (Science and Engineering Research Council, A*STAR, Singapore).

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Fax: +65 6478 9084. Phone: +65 6824 7162

Notes The authors declare no financial interest.

Supporting Information Available: including materials characterization and additional experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

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Oxidative Dehydration of Glycerol to Acrylic Acid over Vanadium

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